Recently, with the rapid development of the mobile communication service, radio portable devices are becoming widespread, and correspondingly low-priced and compact devices have been actively researched.
Radio signals have varying magnitude at any given time due to the distance from the base station and various kinds of obstacles. Therefore, a variable gain amplifier is required to correct this.
There are several variable gain amplifiers, but two typical variable gain amplifiers are shown in FIGS. 1 and 2, respectively.
When the variable gain amplifier shown in FIG. 1 is applied with input voltage through Vip and Vin, the input voltage is converted into the current by transistors M51 and M52, and this current is applied to the load consisting of the current sources I11, I12, and transistor M53, making the output voltage appear at Von and Vop. At this time, if the control voltage Vcon is applied to the gate of the transistor M53, the resistance of the drain-source of the transistor M53 is caused to vary according the control voltage Vcon. That is, the gain can be varied with the variation of the output resistance.
The advantage of the variable gain amplifier shown in FIG. 1 is that the variable gain characteristic at high frequency is excellent, this is because the source connection point C of differential input transistors M51, M52 is virtual ground, and parasitic capacitance C11 of any magnitude generated in the source connection C has non influence on the variable operation.
However, the variable gain amplifier of FIG. 1 has the disadvantage in that case of more than hundreds of mV voltage being applied to the input, a considerable distortion is generated while the input transistor M51 or M52 departs from the conductive state. That is, the variable gain amplifier described above is not able to be used for large input.
In FIG. 2 another type of a variable gain amplifier is shown with the advantage in view of the operational voltage range. The structure of FIG. 2 is similar to that of FIG. 1 except that the transistor M63, which plays a role of active resistor varying with the control voltage Vcon, is connected between the sources of the transistors M61 and M62 forming a pair of inputs.
The variable gain amplifier shown in FIG. 2 operates to raise the control voltage Von and reduce the resistance of transistor M63 when the input voltage is small, thereby to increase the gain. At this time, the distortion is also small since the input is small. Conversely, the variable gain amplifier of FIG. 2 operates to decrease the control voltage Vcon, thereby to increase the source resistance between the input transistors M61, M62 when the input voltage is large. Then, since the input is provided with the negative-feedback due to the large resistance, the gain is reduced and the output is reduced. At this time, the negative-feedback to the large input is caused to reduce the distortion relatively.
However, the variable gain amplifier of FIG. 2 has a problem of gain reduction characteristics at high operating frequency.
Specifically, there is little effect from parasitic elements at low frequency, but a greater effect is generated at high frequency.
In FIG. 2, parasitic capacitive elements C21, C22 exist between the drains and sources of the transistors M63, M64, and M65 and the ground. If the control voltage is reduced due to the large input voltage at the high frequency and the drain-source resistance of the transistor M63 is increased, the source currents of the transistors M61 and M62 gradually flow through the parasitic capacitance elements C21, C22. This has an effect of reducing the impedance in view of the input transistor in spite of no variation of Vcon.
Due to this, the desired gain reduction cannot be obtained. Namely, FIG. 2 shows the variable gain amplifier in which excellent characteristics can be obtained at a low frequency while the desired broad gain reduction characteristics cannot be obtained only at a high frequency. Therefore, the operating frequency region becomes narrowed.